Progress in the brightness, lumen efficacy and affordability of solid state light sources such as light emitting diodes (LEDs) enables new lighting applications that are no longer restricted to niche markets. LEDs offer several advantages over traditional light sources, such as long lifetime, low operating voltage, instant on, etc. For these and other reasons, LEDs are becoming more and more suited for making lamps for several applications such as color variable lamps, spotlights, LCD backlighting, architectural lighting, stage lighting, etc.
For many lighting applications, the light of a single LED is not sufficient, and light of multiple LEDs needs to be combined to form a light source. One solution is to mix light of multiple LEDs in a light guide, before the light leaves the lighting device. An example of such a solution is disclosed in the document “LED Backlight designs using Luxeon high flux light source solutions” by Lumileds, Seattle 2004, http://www.lumileds.com/pdfs/Luxeon_light_source_solutions.pdf. A backlight based on side-emitting LEDs described in this document is schematically illustrated in FIGS. 1a-1b. With reference to FIGS. 1a-1b, the backlight 100 comprises a light guide 102 provided with circular through holes 104b which are arranged in a linear array along an edge 106 of the light guide. In each through hole, there is provided a side-emitting LED 108, whereby light from the LEDs is coupled into the light guide through the sidewall of the through holes, as illustrated by exemplary ray traces 110.
However, when in such a solution the in-coupling holes 104 are closely spaced, it may occur that light from one LED 108a leaves the light guide 102 through a neighboring hole 104 and gets absorbed or scattered at the LED 108b inside this hole. This is illustrated by exemplary ray trace 112 in FIG. 1a. Simulations have shown that for a densely packed linear array of side-emitting LEDs the light leakage is typically about 10% (5% per neighbor). Thus, the lumen efficiency of such a lighting device is degraded.